The present invention relates to a map display system, or more particularly to a map display system in which linear display objects on a map such as roads, rivers and railways are stored as a plurality of point data and these point data are connected by straight lines for display on display means such as a CRT display.
There is a conventional map display system in which for the purpose of guiding the vehicle driver or the like, various map data are stored in advance in memory media such as the magnetic tape, optical or magnetic disc or the ROM package, and the required map data is extracted from them for display on a display such as a CRT.
In this type of map display system, the various map data are stored in the memory medium in one of the two methods mentioned below. One method stores a map as a pattern data and displays the map is displayed as is, in accordance with the pattern data. In the other method, linear objects to be displayed such as roads, rivers, railways, coast lines and boundaries of administrative sections are stored as point data individually, and these points are connected to display these objects in a map. In the former method, the map is displayed in accordance with the pattern data stored, and therefore the processing is effected at a rapid rate. Unless pattern data corresponding to respective scales are prepared for enlargement or reduction, however, the map displayed remains at the same level at the time of enlargement or reduction. This leads to the result that even small roads are displayed after reduction and makes the map hard to read. Therefore a detailed and readable map of the particular map cannot be displayed and readable in this enlarged scale. In the case of displaying a map by enlargement or reduction to various scales, therefore, the latter method is often used. In the latter method, as mentioned above, linear objects to be displayed on the map are stored as point data, which are connected to display a given linear object as a map. Once a display level corresponding to the scale is set together with the coordinate representing the position as point data, roads displayed or not displayed may be determined in accordance with the scale at each time of display, so that the map can be easily enlarged or reduced on the one hand, and the map displayed is very easy to read on the other hand.
The latter method divides each linear object to be displayed at regular intervals of distance, and sets point data for each of the dividing points. As a result, even a substantially rectilinear object of display such as main roads or railways contains a multiplicity of point data, thereby requiring a vast amount of point data. Specifically, a straight object of display may be expressed by a straight line connecting the ends thereof. Nevertheless, a multiplicity of point data are unnecessarily set therebetween in the conventional methods. Also, at the time of display of the map, the unrequired point data are read each time for display, thereby taking a long time for map display.